1. Technical Field
The present invention relates to a control device of an electric power steering apparatus that gives the assist torque by a motor to a steering system of a vehicle.
Furthermore, the present invention relates to a control device of an electric power steering apparatus adapted to give steering assist power to a steering system of a vehicle by driving of a motor, and more specifically, to a control device of a high-performance electric power steering apparatus that makes it possible to surely secure the convergence of the yaw rate of a vehicle.
2. Description of Related Art
In a conventional electric power steering apparatus used for vehicles, the control that a steering torque input by a driver is detected by a torque sensor, and an assist torque according to the steering torque is acquired by a motor, and given to assist steering wheel, thereby reducing the steering power input by the driver is performed. Further, in addition to such control, the electric power steering apparatus detects external environments that surround a vehicle, such as vehicle speed and steering angle, and is mounted with a control device that improves a driver's steering feeling on the basis of these detection results. In particular, the steering angle is used for, for example, steering wheel return control, and the steering wheel return control has such operation that an assist torque is added in a returning direction of a steering wheel according to the steering angle.
As such, the steering angle is a signal that is mainly used for the control of improving a steering feeling in the electric power steering apparatus. As for the steering angle, various methods of detecting the abnormality of a steering angle detector that detects the steering angle, or the abnormality a signal processor that estimates the steering angle, and various countermeasures against this abnormality are known (for example, refer to Reference 1-JP-A-8-54229, Reference 2-JP-A-2003-291842, Reference 3-JP-A-2003-252228, and Reference 4-JP-A-10-274520).
In Reference 1, in order to detect abnormality, such as a disconnection or failure, in a device pertaining to detection or calculation processing of the steering angle, a configuration, in which whether or not a value (relative steering angle value) output by a steering angle sensor, which is a steering angle detection means, falls within a predetermined range, and if the value does not falls within the predetermined range, the value is determined to be abnormality of output, is disclosed.
Further, in Reference 2, a configuration in which, when abnormality of a sensor, such as a steering angle detector that detects a steering angle signal input to a steering wheel return controlling part, has occurred due to disconnection, etc., this abnormality is detected to invalidate steering wheel return control is disclosed.
Moreover, in Reference 3, a configuration in which a case where the deviation between the rotational angle of a motor and steering angle exceeds a comparison reference value is diagnosed as abnormal, and if abnormality is determined, only use of the steering angle for control of a motor current is immediately banned is disclosed. Further, a configuration in which the steering angle is always stored, and if abnormality is determined, a previous steering angle that is stored immediately before the abnormality is determined is used for control of a motor current, and the previous steering angle is gradually reduced, thereby gradually reducing influence on the control of the motor current is also disclosed.
Further, n Reference 4, a steering angle detector in which at least three steering angle sensors that are different in phase from each other according to a change in steering angle are attached to a vehicle, and abnormality of each of the steering sensors is detected from the output state of the steering angle sensor is disclosed i.
Besides, an electric power steering apparatus that auxiliarily energizes (assist) a steering system of a vehicle with the torque of a motor is adapted such that the driving power of a motor auxiliarily energizes a steering shaft or a rack shaft by a transmission mechanism, such as a gear or a belt, via a reduction gear. Such a conventional electric power steering apparatus performs feedback control of a motor current in order to precisely generate an assist torque (steering assist power). The feedback control is control that adjusts a motor-applied voltage so that the difference between a current command value and a motor current detection value may become small. Generally, adjustment of the motor-applied voltage is performed by adjustment of a duty ratio of PWM (Pulse Width Modulation) control.
Here, to explain a general configuration of the electric power steering apparatus with reference to FIG. 13, a column shaft 1002 of a steering wheel 1001 is coupled with tie rods 1006 of steering wheels via a reduction gear 1003, universal joints 1004A and 1004B, and a pinion and rack mechanism 1005. The column shaft 1002 is provided with a torque sensor 1010 that detects the steering torque of the steering wheel 1001, and a motor 1020 that assists in exerting the steering power of the steering wheel 1001 is coupled with the column shaft 1002 via the reduction gear 1003. A control unit 1030 that controls the power steering apparatus is supplied with electric power from a battery 1014, and an ignition key signal from an ignition key 1011, and the control unit 1030 calculates a steering assist command value I of an assist command using an assist map, etc., on the basis of a steering torque value T detected by the torque sensor 1010 and a vehicle speed V detected by a vehicle speed sensor 1012, and controls a current to be supplied to the motor 1020 on the basis of the calculated steering assist command value I.
Although the control unit 1030 is mainly composed of a CPU (or MPU or MCU), the general functions to be executed by a program inside the GPU are as shown in FIG. 14.
To explain the functions and operation of the control unit 30 with reference to FIG. 14, the steering torque T that is detected and input by the torque sensor 1010 and the vehicle speed V from the vehicle speed sensor 1012 are input to the steering assist command value calculating part 1031 Where a basic steering assist command value Iref1 is calculated. The basic steering assist command value Iref1 calculated by the steering assist command value calculating part 1031 is compensated in phase by a phase compensator 1032 for enhancing the stability of the steering system, and a steering assist command value Iref2 that has been compensated in phase is input to an adder 1033. Further, the steering torque T is input to a differential compensator 1035 of a feed forward system for enhancing response speed, and a steering torque TA that has been subjected differential compensation is input to the adder 1033. The adder 1033 adds the steering assist command value Iref2 and a steering torque Td together, and inputs a steering assist command value Iref3 (=Iref2 +Td) that is the result of addition to a subtracter 1034 for feedback.
In the subtracter 1034, a difference Iref4 (=Iref3−i) between the steering assist command value Iref3 and a motor current i that is being fed back is calculated. The difference Iref4 is PI-controlled by a PI controlling part 1036, and is further input to a PWM controlling part 1037 where duty is calculated, and then PWM driving of the motor 1020 is performed via an inverter 1038. The motor current i of the motor 1020 is detected by a motor current detector (not shown), and is input and fed back to the subtracter 1034.
In a control device of such an electric power steering apparatus, a device in which a torsional torque sensor that detects the torsional torque of a steering shaft at the time of turning is provided, and the rotational direction and rotary torque of a motor are controlled according to an output signal of the torsional torque sensor is known as a device that generates moderate response at the time of sudden steering wheel operation (JP-A-45-41246 (Reference 5)).
However, such a control device has a problem in that, if output is set large, the convergence of a steering wheel may degrade due to inertia at the rime of hand-off driving of a vehicle. Further, generally, when a vehicle runs along a sharp curve with sudden operation of a steering wheel, a moderate response to the steering wheel gives a better steering feeling, and there is no means to correct auxiliary steering power (assist power) according to a turning speed. Therefore, when a vehicle runs along a curve having a small radius with sudden operation of a steering wheel, there is a problem in that the steering wheel is too light, which may cause a feeling of anxiety.
As a control device to solve such a problem, there is a control device that applies the brake on the steering angle of a steering wheel, as shown in Japanese Patent No. 2568817 (Reference 6). That is, there is provided a detection means that detects the steering angle speed of a steering system according to a command signal based on an output signal of a torsional torque sensor that detects the torsional torque of the steering system; a steering angle phase compensation command section that issues a damping signal that has determined the rotary torque of a steering wheel in its advancing direction and reverse direction according to the steering angle speed; and a driving controlling part that controls the rotational direction and rotary torque of a motor using as a command signal a signal obtained by adding a command signal based on the damping signal and the torsional torque signal of the steering system. However, since the rotary torque of the steering wheel in its advancing direction and reverse direction is generated in this control device according to a steering angular velocity, and brake is applied to movement of a steering angle, the yawing of a vehicle may diverge, and since the yawing motion and steering angle of a vehicle are not synchronized with each other, a driver's steering feeling become unnatural. Further, since brake is directly applied to movement of a steering angle, there is a problem in that the convergence speed of the steering wheel may become slow, and the vehicle may slip sideways during this period, which is dangerous.
As a means to solve this problem, there is a control device shown in JP-A-2000-95132 (Reference 7). In this control device, the rate of change of the yaw rate of a vehicle is detected, and damping is given to the yaw rate on the basis of the rate of change.
However, the electric power steering apparatuses disclosed in References 1 to 3 have a configuration in which, if it is determined that abnormality has occurred in a device related to detection of a steering angle, some of control is stopped. In this configuration, there are problems in that, when a vehicle is running, a driver's steering feeling may rapidly change due to the stop of control, the driver can not cope with the rapid change, which results in an unstable behavior of the vehicle, and the function of a control system may be reduced, thereby impairing a steering feeling. In particular, in Reference 3, although a configuration in which the steering angle is stored, and the change is gradually performed using a steering angle immediately before abnormality is determined is disclosed, there is no change in that a steering feeling may eventually be impaired, and a steering feeling loss is not suppressed.
Further, in Reference 4, although a configuration in which a plurality of steering angle sensors are attached and abnormality is detected is disclosed, there is a problem in that the configuration may become complicated and the cost may become high.
Further, the control device disclosed in the above Reference 7 performs only the control that the yaw rate is converged by braking, and cannot function to promote the yaw rate when the yaw rate falls due to disturbance. That is, when the steering wheel is released during low-speed driving, etc., the steering wheel is not returned due to disturbance, such as friction, in a case where the yaw rate converge simultaneously when the steering wheel is returned. As a result, there is a problem in that the yaw rate does not converge.